1. Field of the Invention
The present invention relates to a semiconductor device. More specifically, the invention relates to a metal insulator semiconductor field effect transistor (MISFET) having a fin-type structure.
2. Description of the Related Art
A prior art fin-type MISFET (abbreviated as Fin-FET hereinafter) has a structure in which the source/drain easily causes a punch-through phenomenon on the bottom of a fin corresponding to the edge (bottom) of a gate electrode. The Fin-FET therefore has the problem that off-leak currents are high (or short-channel effects are easy to produce). See, for example, Masaki Kondo et al., “A FinFET Design Based on Three-Dimensional Process and Device Simulations,” Toshiba Corporation, IEEE, 2003).
A method of increasing the concentration of impurities of a channel under the fin is proposed in order to resolve the above problem. In other words, a punch-through stopper layer needs to be formed by ion implantation in order to suppress off-leak currents. However, the punch-through stopper layer increases the impurity concentration of the channel. The increase in impurity concentration is likely to degrade the performance of the Fin-FET, e.g., to lower the mobility of electrons, to increase a source-to-drain junction leak (capacitance), and to vary in threshold value.
As described above, the Fin-FET is required to reduce off-leak currents without increasing the concentration of impurities of a channel under a fin.